There have been known a control actuator such as a servo valve which is opened or closed upon receiving a voltage signal or controlled between full opened and full closed state in accordance with the voltage signal used in a plant or equipment as a control means. In such a control actuator, an instruction side to send instruction to the plant or equipment is insulated from a side performing measurement or driving or controlling in the plant side (hereafter referred to as the plant side) depending on use for the purpose of preventing affections of signal voltage on the human body or evading affections of noise. This is generally done by adopting an insulation transformer to transform the voltage of electric power from the power source or by insulating a instruction signal sent from the instruction side to plant side from a signal of control result sent from the plant side to the instruction side by means of an insulation transformer, insulation amplifier, etc.
In recent years, there has been an increased demand for performing soundness diagnosis of circuit in the field of instrumentation and measurement for the purpose of raising reliability of the system by confirming soundness of output signals and circuit wiring, that is, by confirming whether instruction signals are transmitted accurate to the control means and whether there is a breaking of wire or short circuit occurred in the circuit.
FIG. 6 and FIG. 7 respectively show an example of conventional drive circuit of a control actuator and that added with a circuit for diagnosing soundness of the drive circuit. In the figures, reference numeral 101 is an electric power source circuit consisting of an electric power source 102, a pulse generating circuit 103, an insulation transformer 104, a rectifying circuit 105, and a constant voltage circuit 106. A broken line 107 indicates an insulation barrier for insulating the instruction side from the plant side.
First, in a circuit in which soundness diagnosing is not performed as shown in FIG. 6, drive voltage signal 131 of 1.5V, for example, for driving an actuator 130 is modulated into an alternating signal by a modulating circuit 132, amplified by an insulation transformer 133 which is a signal insulation means provided to insulate the plant side from the instruction side, then demodulated by a demodulating circuit 134 supplied with electric power from the electric power source circuit 101, converted by a signal conversion circuit 135 to driving electric power of for example 25V, 4˜20 mmA, and supplied to a driven object 130 such as an actuator, the driven object.
However, with the circuit shown in FIG. 6, a signal of what voltage and current is sent to the actuator which is an object to drive can not be known, and further, although the signal is insulated and sent to the actuator 130, whether the actuator 130 is working normally and whether there is a breaking of wire or short circuit in the wiring to the actuator 130, can not be known.
In a circuit added with a circuit to diagnose the soundness thereof as shown in FIG. 7, the driving voltage signal 131 for driving the actuator 130 which is the driven object is modulated into an alternating signal by the modulating circuit 132 similarly as in FIG. 6, amplified by the insulation transformer 133, demodulated by the demodulating circuit 134, converted by the signal conversion circuit 135 into a voltage signal or current signal corresponding to the driving voltage signal 131 to drive the actuator 130.
In order to confirm the operation of the actuator 130, the driven object, there are provided signal conversion circuits 136a (for current signal) and 136b (for voltage signal) for converting the voltage and current supplied to the actuator 130 from the signal conversion circuit 135, which is supplied with electric power from the electric power source circuit 101, into an alternating signal respectively so as to be able to be inputted to insulation transformers 138a and 138b respectively; modulating circuits 137a and 137b for modulating the outputs from the signal conversion circuits 136a and 136b; and further demodulating circuit 139a and 139b for demodulating the voltages transformed by the insulation transformers 138a and 138b into a current signal and voltage signal. With this circuit configuration, whether operation is performed in accordance with the instruction and whether there is a breaking of wire or short circuit are judged according to whether current is flowing to the actuator 130 and whether the applied voltage is a voltage corresponding to actuation of the actuator 130.
As mentioned above, in the conventional circuit, insulation electric power source circuit 101 including the insulation transformer 104 and constant voltage circuit 106, and an insulation means such as an insulation amplifier composed including the insulation transformer 133 to insulate signals between the plant side and instruction side, are used in order to drive the actuator 130 (driven object) or to drive the signal conversion circuit, modulating circuit, and demodulating circuit. Insulated interface of the instruction side and plant side has been attained in this way. Therefore, in the conventional circuit, an insulation electric power source circuit and insulation amplifiers are needed, which results in a large increase in cost.
In the circumstances, the circuit like this has been able to afford to be applied only in a special case where confirmation of reliability is demanded in spite of increase in cost, because when performing such a soundness diagnosis that whether voltage signals used in controlling the plant and equipment are transmitted accurately and whether there is a braking of wire or short circuit, additional insulation amplifiers are needed to carry out give-and-take of signals between the plant side and instruction side, which requires a large increase of cost.
When performing the soundness diagnosis that whether voltage signals used in controlling the plant and equipment are transmitted accurately and whether there is a braking of wire or short circuit, there occur following problems:                (A) Transmission of signals and supply of electric power must be done by separate circuits.        (B) A modulation circuit and demodulation circuit are needed in order to transmit DC signals via an insulation transformer.        (C) It is required to convert current signals to voltage signals because signal transmission is performed by voltage signals.        
To suffice item (A), it is needed to provide insulation circuits both to the electric power source side and signal transmission side, to suffice item (B), it is needed to provide modulation circuits and demodulation circuits to the signal transmission side, and to suffice item (C), it is needed to provide signal conversion circuits.
As to the art for detect braking of wire, there are disclosed for example in patent literature 1 (Japanese Laid-Open Patent Application No. 2006-023105) a method of detecting breaking of wire by applying a pulse signal to the wire, and comparing the current wave shape measured with the reference current wave shape to judge the presence or absence of breaking of wire from difference in both the wave shapes, and in patent literature 2 (Japanese Laid-Open Patent Application No. 2004-198302) a circuit for detecting breaking of wire by applying a pulse signal for checking via an impedance component to the signal wire for detecting breaking of wire, and comparing the signal obtained from the signal wire with the pulse signal for checking to judge the presence or absence of breaking of wire.
As to diagnosis of electric circuits, there is disclosed for example in patent literature 3 (Japanese Laid-Open Patent Application No. 8-005708) a method of diagnosing electric circuits and diagnosing device used for the method. With which conditions of electric apparatuses are diagnosed for the purpose of improving efficiency of diagnosis operation by facilitating measurement record management and further decreasing occurrence of man-caused errors, by reading out information written and stored in a nonvolatile memory concerning measurement results of characteristics or things concerning measurement of the electric apparatuses, or measurement results of characteristics or things concerning measurement of the electric apparatuses, and comparing the read-out information with the information of-the-moment concerning measurement results of characteristics or things concerning measurement of the electric apparatuses.
However, with the art taught in the patent literature 1 and 2, means for applying pulse signals and a memory for memorizing reference current wave shape are needed, and with the electric circuit diagnosing device disclosed in the patent literature 3, a memory memorized information concerning measurement results of characteristics or things concerning measurement of the electric apparatuses is needed, and further a means for measuring characteristics of the circuit and a means, for comparing the measurement result with the reference data, resulting in complicated composition. Therefore, problems cited in the items (A)˜(C) can not be solved by these art.